1. Field of the Invention
The present invention relates to an inkjet recording apparatus for performing so-called multipass recording, with which an image is recorded on a unit region of a recording medium while a recording head passes over the unit region a plurality of times.
2. Description of the Related Art
In recent years, office automation equipment such as personal computers, copiers, and word processors has become widespread. In order to record images formed by the office automation equipment, recording apparatuses such as inkjet recording apparatuses are used.
An inkjet recording apparatus includes a recording head having a large number of nozzle arrays arranged therein. Each of the nozzle arrays includes ejection nozzles for ejecting ink, which are densely arranged in one direction. This structure is adopted in order to reduce recording time and improve definition of a recorded image. For the purpose of recording a color image, a recording head having inks of different colors and nozzle arrays each corresponding to a color of ink is used.
It is known that, when an inkjet recording apparatus performs recording, the ejection characteristics of a recording head affect recording quality. The accuracy of ejection from the ejection nozzles is affected by even a slight deviation in a manufacturing process of a recording head, which leads to a deviation in the ejection characteristics of the recording head, such as the amount and the direction of ejection from the ejection nozzles. A deviation in the ejection characteristics of the ejection nozzles makes the density of a recorded image uneven and causes degradation of recording quality.
Multipass Printing
In order to reduce the degradation of recording quality, multipass recording has been used. Multipass recording is performed by repeating the process of making a recording head scan a recording medium in a main scanning direction and conveying the recording medium in a sub-scanning direction. In each scan, recording is performed while changing a dividing pattern using a mask or the like. A recording medium is conveyed by a distance shorter than the length of the recording head in the sub-scanning direction. Recording is performed on a region of the recording medium while the region is scanned a plurality of times using different ejection nozzles. With multipass recording, recording is performed in a complimentary manner using ejection nozzles having different ejection characteristics, whereby degradation of recording quality can be reduced.
Connected Heads
In order to increase the recording speed of multipass recording, methods of increasing the width of recording per one scan in the main scanning direction have been used. For example, recording heads have been elongated so as to increase the number of ejection nozzles for one color arranged in the recording heads. That is, nozzle arrays have been elongated in the sub-scanning direction. However, due to technical problems and an increase in cost, it has become difficult to further elongate recording heads in a chip form. Thus, a method of connecting together a plurality of recording heads for one color in the sub-scanning direction has been adopted.
However, when recording heads are connected to one another, disposition of recorded pixels formed on a recording medium may be disturbed by deviation of the installation position of the recording heads and individual differences between the recording heads, which may make image defects such as white lines and black lines conspicuous. Therefore, in an existing technique, a plurality of recording heads are connected to one another in an overlapping manner so that adjacent two recording heads have overlapping portions, and image data is divided between nozzles in the overlapping portions of the two recording heads (see Japanese Patent Laid-Open No. 05-57965).
However, when multipass recording is performed using the overlapping heads having the overlapping portions, if a recording medium is conveyed by a certain distance, the overlapping portions may be used for a recording region a plurality of passes in an overlapping manner. For example, if the multipass recording is performed in N passes, since an image is divided for the overlapping portions in two passes using an overlap dividing mask, a region may exist on which the image is recorded with practically N+1 to 2N passes. If such a region exists, the density of an image in the region is higher that of a region on which an image is recorded with N passes without using the overlapping portions. As the difference in the number of passes becomes larger, the difference in density becomes larger, whereby degradation of an image occurs. In particular, when sub-scanning is performed by a distance (sub-scanning distance) smaller than the width of the overlapping portions (also referred to as “overlapping width”), the overlapping portions are used more frequently and such regions are formed continuously, whereby image degradation becomes conspicuous.